Why is this frame cut off?

I am confused why the Frame in this XAML is being cut off.

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="MobileMidstream.Pages.LoginPage">
    <ContentPage.Content>
        <StackLayout Orientation="Vertical">
            <Frame OutlineColor="Cornsilk"  
                   Padding="10"
                   VerticalOptions="Start"
                   HorizontalOptions="CenterAndExpand">
                    <Label Text="Username:" />
            </Frame>
        </StackLayout>
    </ContentPage.Content>
</ContentPage>

What changes to the XAML would I make to have the Frame anchor to the left side of the screen and fill across to the right side?